This invention relates to the field of lightning protective systems. More particularly, the invention is oriented toward a protective rod discharging electrostatic charges from an aircraft sustaining a lightning strike in-flight. Applications include all types of in-atmosphere craft including military and civilian aircraft as well as earth launched space bound vehicles. The invention has utility in protecting any vehicle suffering a lightning strike where the vehicle and interior equipment cannot be grounded to earth.
Lightning strikes on airborne vehicles are common, especially on aircraft operating at low flight levels such as commercial commuter aircraft and military surveillance and anti-submarine aircraft. The field of aircraft lightning protection and design includes insulators to protect internal electronics, flight control systems and fuel cells, as well as various conductive devices to route the electrical charges away from sensitive areas to heavy structural components comprising aircraft ground.
Lightning strikes and their characteristics have been studied by the National Aeronautics and Space Administration at Langley Research Center for several years. The lightning study project employs an F-106B aircraft specially configured to attract and study lightning strikes. This project has evaluated several hundred in-flight lightning strikes and test results indicate that aircraft lightning damage to aircraft varies depending upon the zone of the aircraft sustaining the strike. Those skilled in the art of lightning strike phenomena study generally divide the aircraft in three zones. Zone one includes areas generally attracting the strike, zone two, the areas on the surface of the aircraft where a high probability exists that the lightning strike will be swept rearward from the attachment point due to the forward momentum of the vehicle and zone three, those areas remaining. The lightning itself is also considered by convention to be categorized into three distinct phases. The prestrike phase where a stepped leader of electrostatic lightning charge is attracted to an attachment point on the aircraft such as the wing tip, nose cone or other salient aircraft projection. When this occurs, the aircraft becomes part of the stepped leader and the stepped leader leaves the aircraft generally in the vicinity of the trailing edges, on its journey to contact earth.
Immediately upon the stepped leader contacting earth a return stroke of extremely high electrical charge transits from earth back to the cloud through the aircraft. This phase is generally referred to as the restrike phase and the lightning restrike return stroke may be as high as 0.2 mega-amps. This lightning restrike is capable of considerable damage to trailing edges and attachment points on an aircraft.
Aircraft with the modern metal and organic matrix composite wings, tails and other control surfaces often sustain heavy damage during this phase of a lightning strike. The damage often takes the form of delamination and/or holes completely burned through the structural surfaces. In some instances considerable loss of matrix material and sometimes complete loss of a static discharge wick in the area of the strike is encountered. In some instances the reinforcing material forming the infrastructure of the area sustaining the strike is disintegrated.
The third phase is a heavy coulomb phase which follows the restrike phase. It is of longer duration and builds up a high coulomb charge. It is also interesting to note that the forward movement of the aircraft tends to move the craft forward out of the lightning channel, forcing the area of the aircraft making the connection to the cloud to sweep back toward the trailing edges of the aircraft. The trailing edges, however, are in the direct conduction path during all three phases of a lightning strike. Consequently, these areas sustain most of the physical damage when an aircraft suffers a lightning strike.
Many devices exist to insulate zone one aircraft parts from becoming an attachment point for the stepped leaders. Other conducting devices such as graphite strips on an aircraft nose cone channel the stepped leader to heavy aircraft structures to minimize damage. Likewise many insulating devices and coatings are employed in zone two of an aircraft to protect interior components from the swept-stroke damage. The zone three areas, unfortunately, lack protecting devices and due to the strike phenomena described above sustain damage during both the stepped leader and restrike phases. Consequently, damage in the form of material decomposition or atomization often occurs on the trailing edges of in-atmospheric craft. This destruction is exacerbated with the relatively non-conducting modern matrix composites.
Accordingly, it is an object of this invention to provide a means of discharging the electrostatic charge passing through the trailing edges of an aircraft, avoiding damage to the vehicle.
Another object of the present invention is to provide protection of trailing edges of an aircraft that can provide protection during all phases of an aircraft lightning strike.
Another object of the present invention is to provide a lightning discharge protection rod that is capable of sustaining multiple lightning strikes while continuing to impart protection.
Yet another object of the present invention is to provide a means of protecting an aircraft from lightning damage that is simple to manufacture and install.
Still another object of the present invention is to provide a means of protecting an aircraft from lightning strike damage in-flight that is inexpensive to manufacture and install.
A further object of the present invention is to provide a means of protection to aircraft parts manufactured from composite materials.
Still another object of the present invention is to provide lightning protection to aircraft parts constructed from organic composites.